Implantable Marker with a Leadless Signal Transmitter Compatible for Use in Magnetic Resonance Devices

ABSTRACT

A leadless marker for localizing the position of a target within a patient. In one embodiment, the marker includes a casing, a resonating circuit, and a ferromagnetic element. The casing is configured to be positioned at a selected location relative to a target site in the patient; the casing, for example, can be configured to be permanently or semi-permanently implanted into the patient. The resonating circuit has an inductor within the casing comprising a plurality of windings of a conductor, but it does not have external electrical lead lines extending through the casing. The ferromagnetic element is at least partially within the inductor. The ferromagnetic element has a volume such that when the marker is in an imaging magnetic field having a field strength of 1.5 T and a gradient of 3 T/m, then the force exerted on the marker by the imaging magnetic field is not greater than gravitational force exerted on the marker.

TECHNICAL FIELD

The present invention is directed toward permanently implantable orsemi-permanently implantable markers with wireless signal transmittersthat are compatible for use in magnetic resonance devices.

BACKGROUND

Medical procedures often require locating and treating target areaswithin a patient. Radiation therapy and many surgical procedures requirelocating the target with a high degree of precision to limit collateraldamage to healthy tissue around the target. It is particularly importantto know or estimate the precise location of the target in radiationoncology because it is desirable to limit the exposure of adjacent bodyparts to the radiation. In applications for treating prostate cancer,for example, the colon, bladder or other body part of the patientadjacent to the prostate is desirably not impinged by the high-intensityradiation beam. Surgical applications, such as breast surgery and otherprocedures involving soft tissue, also require knowing the preciselocation of a target because a lesion is not necessarily fixed relativeto external landmarks on the patient.

Many imaging systems have been used to locate areas or particulartargets within a body before performing radiation oncology or surgicalprocedures. Although x-ray, Magnetic Resonance Imaging (MRI), CT, andother imaging techniques are useful to locate targets within the body atthe pre-operative stage of a procedure, they are often not suitable ordifficult to use in real time during surgery or radiation therapy. Forexample, the location of a lesion in soft tissue or an organ within thepatient's body may shift relative to external landmarks on the patientbetween the pre-operative imaging procedure and the actual radiation orsurgical procedure. Additionally, when imaging systems are used during aradiation or surgical procedure, they may not provide sufficientlyaccurate measurements of the location of the lesions and they mayinterfere with the radiation or surgical procedure. Therefore, imagingtechniques by themselves are not suitable for accurately identifying theactual location of a target for many medical applications.

Another technique to locate a target in a patient is to implant a markerrelative to the target. For example, implantable markers that generate asignal have been proposed for use to locate a selected target in apatient in radiation oncology procedures. U.S. Pat. No. 6,385,482 B1issued to Boksberger et al. discloses a device having an implantedemitter unit SE located inside or as close as possible to a targetobject T and a plurality of receiver units S11, S12, S21 and S22 thatare located outside of the patient. Boksberger discloses determining thelocation of the target object T by energizing the emitter unit SE usinggenerator GE and sensing the signal from the emitter unit SE with thereceiver units S11-S22. Boksberger discloses and claims that thereceiver units S11-S22 are configured to determine the gradient of themagnetic field generated by the emitter unit SE. Boksberger disclosesemitter units SE that are energized using a wired connection to theexternal generator GE. Boksberger also indicates that it is conceivableto use an emitter unit SE that is energized by a battery or excited byan electromagnetic field generated by the external generator GE. Thewired device disclosed in Boksberger, however, may not be suitable foruse in radiation oncology and many surgical procedures because it isimpractical to leave a wired marker implanted in a patient for theperiod of time of such procedures (e.g., five to forty days). Moreover,Boksberger does not disclose or suggest anything with respect toproviding an implantable emitter unit SE that is compatible for use inmagnetic resonance imaging devices after being implanted in a patient.

Another technique to locate a target in a patient is to implant passive,gold fiducials in or near the target site. The positions of the goldfiducials are determined periodically using radiation. Although goldfiducials are useful for localizing a target within a patient, thesesystems do not provide sufficiently accurate real time measurements ofthe target site location during radiation oncology procedures.

Other types of tags or markers with resonating magnetic circuits havebeen developed. These markers have been used to tag sponges and otheritems used during surgery or locate the general location of feedingtubes or other instruments in other procedures. One significantchallenge of miniature, wireless markers is to provide a sufficientlystrong signal to be accurately detected by sensors outside of the body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of an implantable wireless marker inaccordance with an embodiment of the invention with a section cut awayto illustrate internal components.

FIG. 2 is a cross-sectional view taken along a longitudinal axis of anembodiment of the marker of FIG. 1.

FIG. 3 is a cross-sectional view in a plane normal to a longitudinalaxis of a marker in accordance with an embodiment of the marker shown inFIG. 1.

FIG. 4 is a cross-sectional view taken along a longitudinal axis of amarker in accordance with an embodiment of the invention after beingimplanted in a patient.

FIG. 5 is a diagram of a display of a magnetic resonance image with anartifact by a magnetic marker.

FIG. 6 is a cross-sectional view taken along a longitudinal axis of amarker in accordance with another embodiment of the invention.

DETAILED DESCRIPTION

The following disclosure describes several embodiments of wirelesslyenergized markers configured to be implanted in or externally attachedto patients for a long period of time and compatible for use in powerfulmagnetic fields generated by magnetic resonance imaging devices. Severalembodiments and features of markers in accordance with the invention areset forth and described in FIGS. 1-6. It will be appreciated that otherembodiments of markers in accordance with the invention can includeadditional or different features than those shown in FIGS. 1-6.Additionally, it will be appreciated that several embodiments of markersin accordance with the invention do not include all of the featuresshown in these figures. Additionally, for purposes of brevity, likereference numbers refer to similar or identical components.

FIG. 1 is an isometric view of an implantable marker 100 in accordancewith an embodiment of the invention with a portion cut away toillustrate internal components.

The embodiment of the marker 100 shown in FIG. 1 includes a casing 110and a resonating circuit 120 in the casing 110. The casing 110 is abiocompatible barrier configured to be implanted in the patient orotherwise attached to the patient. The casing 110 can be a generallycylindrical capsule that is sized to fit within a 14 gauge needle forpercutaneous implantation, but the casing can have other configurationsand be larger or smaller. The casing 110, for example, can have barbs toanchor the casing 110 in soft tissue or an adhesive for attaching thecasing 110 externally to the skin of a patient. In one embodiment, thecasing 110 includes (a) a glass capsule or shell 112 having a closed end114 and an open end 116, and (b) a sealant 118 in the open end 116 ofthe shell 112. The casing 110 and sealant 118 can be made from plastics,ceramics, glass or other suitable biocompatible materials.

The resonating circuit 120 produces a wirelessly transmitted signal inresponse to a wirelessly transmitted excitation signal. In oneembodiment, the resonating circuit 120 comprises a coil 122 defined by aplurality of windings of a conductor 124. Many embodiments of theresonating circuit 120 also include a capacitor 126 coupled to the coil122. The coil 122 resonates at a selected resonant frequency. The coil122 can resonate at the selected resonant frequency solely using theparasitic capacitance of the windings without having a capacitor, or theselected resonant frequency can be produced using the combination of thecoil 122 and the capacitor 126. The coil 122 by itself or in combinationwith the capacitor 126 accordingly defines a signal transmitter thatgenerates an alternating magnetic field at the selected resonantfrequency in response to the excitation signal. The conductor 124 of theillustrated embodiment can be hot air or alcohol bonded wire having agauge of approximately 45-52 gauge. The coil 122 can have 800-2000turns. The windings are preferably wound in a tightly layered coil.

The resonating circuit 120 is powered by a wirelessly transmittedexcitation signal such that the resonating circuit is leadless, i.e.,not connected to external lead wires which extend through or projectfrom the casing 110. In one embodiment, the resonating circuit 120 canbe energized by an alternating excitation magnetic field generatedexternally with respect to the patient at the resonant frequency of theresonating circuit. In response to the excitation field, the resonatingcircuit 120 produces a marker signal or response signal that can bemeasured by a sensor array positioned externally with respect to thepatient. Suitable devices for generating the magnetic excitation fieldand sensing the marker signal are disclosed in U.S. patent applicationSer. Nos. 10/027,675 filed on Dec. 20, 2001; 10/044,056 filed on Jan.11, 2002; and 10/213,980 filed on Aug. 7, 2002, which are hereinincorporated by reference.

FIG. 2 is a cross-sectional view of an embodiment of the marker 100taken along a longitudinal axis 2-2 shown in FIG. 1. The marker 100further includes a ferromagnetic element 140 having a first end 142 anda second end 144. The ferromagnetic element 140 is at least partiallysurrounded by the coil 122. In the particular embodiment shown in FIG.2, the coil 122 surrounds the ferromagnetic element 140 from the firstend 142 to the second end 144. In other embodiments, the coil 122surrounds only a portion of the ferromagnetic element 140. The capacitor126 can be positioned at the first end 142 of the ferromagnetic element140. Additionally, the resonating circuit 120 and the ferromagneticelement 140 can be fixed to the casing 110 by an adhesive 150.

The ferromagnetic element 140 is preferably composed of ferrite or othermaterials that have high magnetic permeability compared to free space.The amount of energy that the inductor is capable of storing is limited,in part, by the magnetic field saturation of the ferromagnetic element140. To store more energy in a miniature wireless marker, the prior arttaught that the size of the ferromagnetic material should be maximizedwithin the limited space of the marker. As shown in FIG. 2, however, thevolume of the ferromagnetic element 140 is significantly less than theavailable volume within the casing 110. The smaller volume of theferromagnetic element 140 reduces the force exerted on the marker 100when the marker 100 is placed in a magnetic resonance imaging devicehaving a magnetic field strength of 1.5 T with a corresponding gradientfield of approximately 3 T/m. In one embodiment, the ferromagneticelement has a volume such that when the marker is in a magneticresonance device, then the force exerted on the marker by the magneticfield is less than gravitational force exerted on the marker.Additionally, the small volume of the ferromagnetic element 140 reducesthe size of the artifact in an image from a magnetic resonance device.It will be appreciated that ferromagnetic materials will produce anartifact (i.e., a region in which image information is suppressed) in animage produced by a magnetic resonance imaging device. The volume of theferromagnetic element 140 can be reduced to a size such that it producesa small artifact in an image from a magnetic resonance device. Ingeneral, such ferromagnetic elements 140 have small diameters less thanthe size of commercially available ferrite rods for transponderapplications, which are as small as 0.75 mm in diameter (i.e., ferriterods available from Ferroxcube of Spain).

FIG. 3 is a cross-sectional view of the marker 100 taken along line 3-3of FIG. 2. In one embodiment, the ferromagnetic element 140 is a ferriterod having a diameter D, of approximately 0.20-0.70 mm, but theferromagnetic element 140 can have other cross-sectional configurationsin other embodiments. For example, an extruded ferrite rod can have anelliptical, oval or polygonal cross section. The ferromagnetic element140 can have a length of approximately 2.0-20 mm. In one particularembodiment the ferromagnetic element 140 has a diameter of approximately0.25-0.50 mm and a length of 2-12 mm, and in another embodiment theferromagnetic element 140 has a diameter of 0.30-0.35 mm and a length of4.0-6.0 mm. The coil 122 has an inner diameter of approximately0.20-0.80 mm and an outer diameter D₂ of approximately 0.6-1.4 mm or0.8-1.9 mm. The casing 110 can have an outer diameter D₃ ofapproximately 1.0-3.0 mm. In other embodiments, the coil 122 can havedifferent inner and outer diameters, and the casing 110 can have adifferent outer diameter. In another particular embodiment, the diameterD₁ of the ferromagnetic element 140 is approximately 0.30-0.50 mm, theinner diameter of the coil 122 is approximately 0.30-0.60 mm, the outerdiameter D₂ of the coil 122 is approximately 1.2-1.9 mm (or 1.2-1.4 mm),and the outer diameter D₃ of the casing 110 is approximately 1.8-2.0 mm.The volume of the ferromagnetic element 140 can be approximately0.5-19.0 mm³.

The marker 100 is constructed by manufacturing the ferromagnetic element140, placing the coil 122 around the ferromagnetic element 140, andencapsulating the resonating circuit 120 and the ferromagnetic element140 in the casing 110. The ferromagnetic element 140 can be manufacturedusing extrusion, coring, or high pressure molding processes to form aferrite rod having a diameter of approximately 0.2-0.7 mm. The coil 122is formed by winding the conductor 124 around either the ferromagneticelement 140, a sleeve around the ferromagnetic element 140, or a mandrelseparate from the ferromagnetic element 140. In one embodiment, theconductor 124 is wrapped directly onto the ferromagnetic element 140,but this may not be feasible in many applications because it may breakferromagnetic elements having a diameter less than 0.5 mm. In anotherembodiment, a retractable sleeve can slide along the ferromagneticelement 140 as the conductor 124 is wound directly onto theferromagnetic element. The sleeve is expected to support theferromagnetic element 140 as the first layer of turns are wrapped aroundthe ferromagnetic element 140. The first layer of turns supports the rodso that subsequent layers of turns can be wound onto the first layer. Instill another embodiment, the coil 122 is wound around a mandrelseparately from the ferromagnetic element 140. The coil 122 is thenremoved from the mandrel and the ferromagnetic element 140 is insertedinto the inner diameter of the coil 122. This embodiment can result in asmall gap between the ferromagnetic element 140 and the inner diameterof the coil 122. This gap should be minimized in optimal circumstancesto increase the performance of the resonating circuit 120. After theferromagnetic element 140 is positioned within the coil 122, thisassembly is adhered to the casing 110 using the adhesive 150, and thesealant 118 is used to close the open end 116 of the casing 110.

FIG. 4 is a representative view of the operation of the marker 100 in anmagnetic field M generated by a magnetic resonance imaging device (notshown). The magnetic field M is an imaging magnetic field. In thisembodiment, a patient is placed in a magnetic resonance imaging deviceto image a portion P of the patient. The imaging magnetic field Mincludes a plurality of flux lines F. Because the ferromagnetic element140 has a high magnetic permeability, the ferromagnetic element 140exerts a magnetic force F_(M) in the presence of the magnetic field Mdue to the presence of DC and gradient magnetic fields. The magnitude ofthe magnetic force F_(M) is a function of the volume and the type ofmaterial (i.e. magnetic saturation) of the ferromagnetic element 140.The volume of the ferromagnetic element 140 is selected so that themagnetic force F_(M) caused by the interaction between the ferromagneticelement 140 and the magnetic field M is less than the gravitationalforce F_(G) exerted against the marker 100. This will ensure that themagnetic field M does not cause the marker 100 to move within theportion P of the patient any more than the force of gravity will causemovement of the marker 100.

FIG. 5 is a schematic representation of a magnetic resonance image 500that shows a target location T within a body part of a patient. Theimage 500 includes an artifact 510 caused by the ferromagnetic element140 of the marker 100. The artifact 510 is typically much larger thanthe size of the marker, and thus it tends to obscure the actual locationof the marker and the images of tissue adjacent to the marker. The sizeof the artifact 510 is related to the size of the ferromagnetic element140 in the marker 100. In several embodiments, the volume of theferromagnetic element 140 is selected to produce an artifact not greaterthan 1,500 mm² in an image produced by a resonance imaging device fieldhaving a DC field strength of 1.5 T. In other embodiments, the volume ofthe ferromagnetic element 140 is selected to produce an artifact notgreater than 400-1,200 mm², and in other cases not greater than 400-800mm² in an image produced by a magnetic resonance imaging device fieldhaving a DC field strength of 1.5 T.

FIG. 6 is a cross-sectional view of a marker 600 in accordance withanother embodiment of the invention. The marker 600 is substantiallysimilar to the marker 100 shown in FIG. 2, but the marker 600 furtherincludes a module 610 at the second end 144 of the ferromagnetic element140. The module 610 is preferably configured to be symmetrical withrespect to the capacitor 126 at the first end 142 of the ferromagneticelement 140. The module 610, more specifically, is configured to producea similar radiographic image as the capacitor 126 in an x-ray. In oneembodiment, the module 610 is configured such that the magnetic centroidof the marker is at least substantially coincident with the radiographiccentroid of the marker. In other embodiments that use CT or other typesof imaging modalities, the module 610 is configured to produce asymmetrical image relative to the capacitor 126. For example, the module610 can be another capacitor identical to the capacitor 126 that may ormay not be electrically coupled to the coil 122. In other embodiments,the module 610 can be an electrically inactive element that is notelectrically connected to the resonating circuit 120 or another type ofelectrically active element that is electrically coupled to theresonating circuit 120. Suitable electrically inactive modules includeceramic blocks shaped like the capacitor 126. In either case, onepurpose of the module 610 is to have the same characteristics as theelectrically active capacitor 126 in x-ray, CT, and other imagingtechniques. Since the markers may be located via radiographic methods(e.g. CT, or x-ray) to determine the marker centroid positions relativethe target tissue prior to therapy, an error in the position of themarker radiographic and magnetic centroids may result in a fixedpositional error during therapy.

From the foregoing, it will be appreciated that although embodimentshave been described for purposes of illustration, various modificationsmay be made without deviating from the spirit and scope of theinvention. Accordingly, the invention is not limited except by theappended claims.

1-40. (canceled)
 41. A method for imaging a target within a patient thathas a marker implanted relative to the target, comprising: subjectingthe patient to a magnetic field in a magnetic resonance imaging device;and providing a ferrite core in the marker having a volume such thatforce exerted on the marker by the magnetic field is less thangravitational force exerted on the marker.
 42. The method of claim 41,wherein providing a ferrite core comprises placing a ferrite rod havinga diameter not greater than approximately 0.5 mm in a coil having aplurality of turns, and inserting the ferrite rod and coil into a casinghaving an outer diameter not greater than approximately 2 mm.
 43. Themarker of claim 41 wherein providing a ferromagnetic core comprisesplacing a ferrite rod having a diameter of approximately 0.2 mm-0.7 mmand a length of approximately 2 mm-12 mm in a coil.
 44. The marker ofclaim 41 wherein providing a ferromagnetic core comprises placing aferrite rod having a volume of approximately 0.5 mm3-19.0 mm3 into acoil.
 45. A method of manufacturing a marker, comprising: providing aferromagnetic element; positioning a coil of an inductor at least arounda portion of the ferromagnetic element, wherein the coil comprises aplurality of windings of a conductor; and encasing the ferromagneticelement and the coil in a casing, wherein the ferromagnetic element hasa volume such that when the marker is in a magnetic resonance devicehaving a field strength of 1.5 T and a gradient of 3 T/m, then forceexerted on the marker by the magnetic field is less than gravitationalforce exerted on the marker.
 46. The method of claim 45 wherein theinductor further comprises a capacitor electrically coupled to theconductor and the casing comprising a barrier having an outer diameterof not greater than approximately 2.0 mm, and wherein providing theferromagnetic element comprises providing a ferrite rod having adiameter not greater than approximately 0.5 mm.
 47. The method of claim45 wherein providing the ferromagnetic element comprises providing aferrite rod having a diameter of approximately 0.2 mm-0.7 mm and alength of approximately 2 mm-12 mm.
 48. The method of claim 45 whereinproviding the ferromagnetic element comprises providing a ferrite rodhaving a volume of approximately 0.5 mm3-19.0 mm3.
 49. The method ofclaim 45 wherein providing the ferromagnetic element comprises providinga ferrite rod having a volume less than a volume of ferrite thatproduces an artifact of 1500 mm2 in an image produced by a resonatingmagnetic field of approximately 1.5 T with a field gradient of 3 T/m.50. The method of claim 45 wherein providing the ferromagnetic elementcomprises providing a ferrite rod having a volume less than a volume offerrite that produces an artifact of 400 mm2-1200 mm2 in an imageproduced by a resonating magnetic field of approximately 1.5 T with afield gradient of 3 T/m.
 51. The method of claim 45 wherein the windingscomprise a coil having an inner diameter of approximately 0.2 mm-0.8 mmand an outer diameter of approximately 0.8 mm-1.9 mm and the casing hasan outer diameter of approximately 1.5 mm-2.5 mm, and wherein providingthe ferromagnetic element comprises providing a ferrite rod having adiameter of approximately 0.2 mm-0.7 mm.
 52. The method of claim 45wherein the windings comprise a coil having an inner diameter ofapproximately 0.3 mm-0.6 mm and an outer diameter of approximately 1.2mm-1.9 mm and the casing has an outer diameter of approximately 2 mm,and wherein providing the ferromagnetic element comprises providing aferrite rod having a diameter of approximately 0.3 mm-0.5 mm, 53.(canceled)
 54. A method of manufacturing a marker comprising: providinga ferromagnetic element; positioning a coil of an inductor at leastaround a portion of the ferromagnetic element, wherein the coilcomprises a plurality of windings of a conductor; and positioning theferromagnetic element such that the radiographic and magnetic centroidsof the marker are at least substantially coincident.